This invention relates generally to
(a) a method for heating a hydrocarbon-containing subterranean formation to develop a zone containing carbonized material in the pore spaces and PA1 (b) the mineral formation containing carbonized material in the pore spaces resulting from the heating process. PA1 (a) forming a borehole in the hydrocarbon-bearing formation, PA1 (b) placing a heating device in said borehole, PA1 (c) energizing the device to heat the surrounding formation to a temperature high enough to produce coking of at least a portion of the hydrocarbon-bearing formation, and PA1 (d) maintaining the temperature of step (c) for a length of time to obtain the current-carrying electrode of desired radius.
More particularly, the invention relates to a method for creating a carbonaceous current-carrying deposit in a formation surrounding a borehole, and to the enlarged-radius electrode thus formed from the deposit. A borehole that is completed as a well and having appropriate electrical features so that it can function as an electrode in contact with the adjacent formation is known as an electrode well. The utility of the invention lies in the heating, by electrical means, of a subterranean formation, between two or more boreholes, as the step following the formation of the carbonaceous electrode.
Broadly, when an electrical current is used in a subterranean formation to heat the formation, it is desirable to have an electrode of substantial size. If small electrodes are used, a high current density develops, which leads to a high temperature in the vicinity of the electrode. This high temperature vaporizes or flashes the connate brine or water, with said flashing effectively removing some of the electrolyte present, thus reducing the conductivity and even leading to an interruption of the process. The flash temperature depends on the depth of the electrode and, broadly, can vary from about 220.degree. to about 600.degree. F. (104.degree.-315.degree. C.). In an effort to overcome the problem of flashing, and thus the reduction in electrical conductivity, previous schemes have suggested injecting metal or graphite particles into the formation to keep the current path open and reduce the current density, thus delaying the onset of the flashing phenomenon. U.S. Pat. No. 3,848,671 (Kern) concerns a method of producing bitumen in which injection and production wells are completed, and the formation is heated by passing electricity between electrodes positioned in each well. As mentioned above, the Kern process has the limitation that during heating, the temperature immediately adjacent the wells must not be so high as to cause evaporation of the water envelopes, at the pressure found in the formation. U.S. Pat. No. 3,958,636 (Perkins) produces bitumen from a tar sand formation while heating the formation by electrical conduction between a plurality of wells. A high back pressure is maintained on the wells and an immiscible fluid is injected into the formation through one of the wells. However, like the above Kern patent, Perkins discloses that during heating, the temperature in the regions of highest current densities, that is, in the regions immediately about and adjoining the wells, should not be so high as to cause evaporation of the water envelopes at the pressure that is sustainable by the overburden. This means that the electrical current should be maintained low enough to prevent drying of the tar sand formation around the wells. U.S. Pat. No. 3,931,856 (Barnes) increases the "size" of the electrode used in heating by providing a larger area of high electrical conductivity. This is done by having an electrode well adjacent a satellite well. Preliminary heating of the formation between these wells mobilizes the viscous oil, and it is removed. Then, water containing an electrolyte is circulated between the electrode and satellite wells, effectively increasing the "size". U.S. Pat. No. 3,874,450 (Kern) enlarges an electrode by having an upper section of conductive casing in a vertical wellbore with a lower section of nonconductive casing. The bottom of the wellbore has a deviated section extending laterally from the vertical axis of the bore in a predetermined direction. This deviated section contains an electrode and is filled with electrolyte. When electricity is applied to the wellbore, current flows between the upper section and the deviated section, thus heating the formation over a larger volume than is possible by prior methods. This deviation operation necessitates additional drilling variables and complicates the wellbore completion, resulting in additional expense. The Kern '671 and Perkins methods are careful to point out that, during formation heating, the temperatures adjacent the electrode wells must not be so high as to cause evaporation of the water envelopes.